The present invention relates generally to manufacturing, and, more specifically, to machining individual workpieces.
A gas turbine engine compressor includes a row of compressor rotor blades or airfoils extending radially outwardly from a supporting disk. Each blade may include an integral dovetail for removably mounting the blade to the perimeter of a disk having a complementary dovetail slot therein. Or, the blades may be integrally formed with the disk in a one-piece or unitary bladed-disk assembly commonly referred to as a blisk.
Each blade has an airfoil configuration with a generally concave pressure side and an opposite generally convex suction side extending radially in span from root to tip between axially spaced apart leading and trailing edges. The airfoil has a complex three-dimensional (3D) configuration and typically is twisted about its radial stacking axis.
Disk blades or blisk blades may be manufactured in various manners, and with different degree of difficulty and expense. In either case, material must be removed from an initial workpiece to achieve the desired configuration of the airfoil in accordance with its design specification.
The configuration of each airfoil is typically defined by a suitable number of surface points in a three-dimensional coordinate system. The nominal configuration of each blade specifies its desired aerodynamic shape and relative position of its features.
Since all manufacturing processes are subject to random variations in material removal, the nominal configuration is bounded by suitably small tolerances of larger and smaller variations in dimensions which are acceptable for a particular design. For example, the dimensions of a particular component design may vary up to a few mils either greater than or less than the nominal dimension for the desired configuration.
Accordingly, during the manufacture of individual blades the final configuration thereof is never exactly the same as the nominal configuration but varies within the permitted tolerances over the entire outer surface of the component. Since a given rotor stage has a considerable number of blades around the perimeter of the supporting disk, no two blades will be identically alike although all the blades will be formed within the permitted tolerances of the nominal configuration.
Random variations in the final configuration of the blades will occur irrespective of the particular method of forming the blades. For example, electrochemical machining (ECM) erodes material from a workpiece using a pair of cathode electrodes having contours complementary with the desired side contours of the blade.
Individual blades may also be formed using a numerically controlled milling machine in which the nominal configuration of the blade is stored in a suitable coordinate system and the cutting tool follows a corresponding cutting path around the workpiece for forming the final configuration thereof.
In both examples described above, the individual blades are formed using the nominal configuration thereof subject to the permitted tolerances or variations in final surface dimensions.
Gas turbine engine blades are typically manufactured from high strength materials with a high degree of accuracy reflected by relatively small manufacturing tolerances, and thusly the cost of production is relatively high. A blade may be damaged during the manufacturing process or during use in the gas turbine engine and it is therefore desired to repair that blade for preventing the wasteful scrapping thereof.
For a rotor disk having removable blades this is less of a problem since an individual blade may be more readily repaired remote from the disk or simply substituted with another blade. However, for a blisk having integral blades, a damaged blade must be repaired in situ since otherwise the entire blisk including its many blades is subject to scrapping.
In a recent development program, the weld repair of titanium blisks for a gas turbine compressor application is being explored. Damage to the relatively thin leading or trailing edges of an individual blade may be repaired by removing the damaged portion and weld repairing the remaining cutout. Either weld material may be built up in the cutout, or a suitable spad insert may be welded therein.
In either case, the weld repair is intentionally made larger than the nominal configuration of the blade so that the repair may be subsequently blended with the original blade contour. Since the blade is an aerodynamic component, a smooth surface thereof is required without steps or discontinuities which would adversely affect aerodynamic performance. The weld repair may be manually blended to shape using a grinder, for example, but is subject to corresponding inaccuracies.
Machine blending of the weld repair is desired but the inherent variation of the configuration of an individual blade from the nominal configuration introduces an additional uncertainty in the machining process which will cause either insufficient or excessive machining at the weld repair relative to the undamaged adjacent surfaces, and resulting discontinuities or steps therebetween.
Accordingly, it is desired to provide a process for re-machining a pre-machined workpiece to a nominal configuration within the originally specified tolerances therefor.
The nominal configuration of a workpiece is stored in a multiaxis numerically controlled machine. The workpiece is probed in the machine to determine an offset from the nominal configuration. The workpiece is then shifted by the offset to correspond with the nominal configuration. The workpiece is then machined according to the nominal configuration.